1. Field of the Invention
The present invention relates to a process for the preparation of a phosphor, and more particularly, to a process for the preparation of a divalent europium activated alkaline earth metal fluorohalide phosphor containing a metal oxide.
2. Description of the Prior Art
A divalent europium activated alkaline earth metal fluorohalide phosphor absorbs a radiation such as X-rays with high efficiency and, when exposed to such a radiation, emits near ultraviolet to blue light of high luminance (spontaneous emission). Accordingly, it has been known that said phosphor is employable in a radiographic intensifying screen used in medical radiography for medical diagnosis and in industrial radiography for nondestructive test. Recently, it has been discovered that the divalent europium activated alkaline earth metal fluorohalide phosphor can serve as a stimulable phosphor. That is, when exposed to a radiation such as X-rays, the phosphor absorbs and stores a portion of the radiation energy and emits near ultraviolet to blue light when excited with an electromagnetic wave within a wavelength region of 400-850 nm after exposure to the radiation. This emission is called "stimulated emission". Because of the stimulability thereof, the divalent europium activated alkaline earth metal fluorohalide phosphor has been paid much attention and investigated as a phosphor of a radiation image storage panel employable in a radiation image recording and reproducing method utilizing a stimulable phosphor, for example, as disclosed in Japanese Patent Provisional Publication No. 55(1980)-12429.
When a radiation image storage panel is employed for carrying out medical diagnosis, it is particularly desired that the sensitivity to a radiation of the panel is as high as possible to decrease the exposure dose for patient and to facilitate the procedure for converting the stimulated emission to electric signals. Accordingly, it is desired that the luminance of stimulated emission of the phosphor used for the panel is improved as high as possible. Especially when the radiation is applied to a human body as described above, the enhancement in the luminance of stimulated emission is of much value from the viewpoint of adverse effect of the radiation on the human body. Accordingly, in that case, the enhancement in the luminance is particularly desirable even if the level of the enhancement is not so remarkable.
The divalent europium activated alkaline earth metal fluorohalide phosphor is generally prepared by a process comprising steps of mixing starting materials for a phosphor and firing the resulting mixture for an appropriate period of time at a high temperature. In this process, there is a problem that the resulting phosphor is apt to sinter in the firing step. For this reason, supplemental procedures for pulverizing the fired product and classifying the so pulverized product are needed after the firing step. However, the supplemental pulverizing and classifying procedures remarkably reduce the yield of the phosphor. Further, in the preparation of a phosphor, the sintering phenomenon caused in the firing step generally gives adverse effect to the emission characteristics, particularly to the emission luminance, of the phosphor to be obtained. Accordingly, in the preparation of the divalent europium activated alkaline earth metal fluorohalide phosphor, it is considered that the sintering phenomenon in the firing step likewise gives an adverse effect to the luminance of stimulated emission of the phosphor.
As a method for preventing the sintering phenomenon in the firing step in the process for the preparation of a phosphor, it has been known to add to the mixture of starting materials for the phosphor a metal oxide or a metal compound which can be easily converted into a metal oxide at a high temperature. The application of the above-mentioned method to the preparation of a divalent europium activated alkaline earth metal fluorohalide phosphor has been already known, and accordingly, the divalent europium activated alkaline earth metal fluorohalide phosphor containing a metal oxide prepared by the method is known.